Gas permeation. Because it plays a central role in this invention the concept of gas permeation is presented here.
Without continual or periodic pumping down, the initial low pressure of any vacuum contained in a vessel will increase as atmospheric gas permeates through the materials of which the vessel is made. The rate of pressure increase will depend on the rate of permeation. Therefore the service life of a vacuum insulating glass (VIG) unit is not indefinite but can be extended, provided there is not a failure of the edge seal, by periodic pumping down through a permanently attached or temporarily attachable pump out port.
With regard to permeation Roth (1994, p 6-7) states (references cited: other publications):                Gases have the possibility to flow through solids even if the openings present are not large enough to permit a regular flow. The passage of a gas into, through and out of a barrier having no holes large enough to permit more than a small fraction of the gas to pass through any one hole is known as permeation. The steady state rate of flow in these conditions is the permeability coefficient or simply permeability. This is usually expressed in cubic centimeters of gas at STP [standard temperature and pressure] flowing per second through a square centimetre of cross section, per millimetres of wall thickness and 1 torr of pressure drop across the barrier . . . . An ideal vacuum should maintain forever the vacuum (pressure) reached at the moment of its separation from the pumps. Any real chamber presents a rise in pressure after being isolated from the pumping system. The pressure rise is produced by the gas which permeates from outside into the chamber . . . .        
Also in regard to permeation O'Hanlon (2003, p 70) states (references cited: other publications):                Permeation is a three step process. Gas first absorbs on the outer wall of a vacuum vessel, diffuses through the bulk, and lastly desorbs from the interior wall. Permeation through glass, ceramic, and polymeric materials is molecular. Molecules do not dissociate on absorption. Hydrogen does dissociate on metal surfaces and diffuses as atoms that recombine before desorption on the vacuum wall.        
Ceramic glasses typically used for VIG units have permeability to atmospheric gases in the range of 10−12 to 10−13 cm3·mm/(cm2·sec·torr).
Vacuum Insulating Glass Units. Vacuum insulating glass units are known in the art. For example, see U.S. Pat. Nos. 5,664,395; 5,657,607; 5,891,536; 5,902,652; 6,444,281 B1; 6,291,036; and 7,141,130 B2 the disclosures of which are all hereby incorporated herein by reference.
Vacuum insulating glass (VIG) units comprise two substantially parallel spaced apart glass sheets with a vacuum in between at a pressure less than atmospheric pressure. Between the glass sheets are visually nonintrusive spacers that maintain the vacuum space by resisting compressive atmospheric pressure. Common to all VIG units is an edge seal that seals the edge gap between the glass sheets and maintains the vacuum by presenting a low permeability barrier.
Thermal heat transfer via convection and conduction cannot occur through a vacuum. Consequently the energy and associated cost savings that can result from the use of VIG units in applications such as windows, doors, and skylights can be on the order of ten times greater than for inert gas filled thermal pane units, which have an inert gas such as argon or krypton at atmospheric pressure between their glass sheets.
There are serious unresolved performance and reliability problems that continue to hamper development of commercially viable VIG units, forestalling the significant energy savings that will result should they ever replace inert gas filled thermal pane. Chief among them is edge seal failure and sudden brittle fracture of the relatively non-ductile glass sheets. These failures are caused by large stresses resulting from differential thermal expansion and contraction (or “differential thermal strain”) of the thermally separated glass sheets. The patent record reveals an ongoing intensive effort to solve this problem by employing more flexible edge seal designs. The effort is spurred by a quest to capitalize on market demand for more energy efficient buildings. The demand is driven by a pressing need to forestall the mounting dangers of global warming by reducing green house gas emissions.
Steven Chu, Secretary, U.S. Department of Energy, stated at the Caltech Commencement, Jun. 12, 2009:                There is a growing realization that we should be able to build buildings that will decrease energy use by 80 percent with investments that will pay for themselves in less than 15 years. Buildings consume 40 percent of the energy in the U.S., so that energy efficient buildings can decrease our carbon emissions by one third.        
At the time of this writing, residential buildings account for 22 percent of U.S. energy consumption, commercial 18 percent. Of the 22 percent residential energy consumption, 42 percent is a result of residential heating and cooling. Buildings use 72 percent of the nation's electricity and 55 percent of its natural gas. Buildings are responsible for approximately 40 percent of CO2 emissions in the U.S., and approximately 2,300 teragrams (Tg or million tonnes (MMT)) CO2 equivalent (source U.S. Department of Energy).
The U.S. Green Building Council has instituted an internationally recognized green building certification system known as Leadership in Energy and Environmental Design or LEED certification that promotes energy savings, water efficiency. CO2 emissions reduction, and improved indoor environmental quality. Leed standards promote greater use of natural light and visibility to the outdoors. VIG units make this possible without being at cross purposes with LEED energy saving and CO2 emissions reduction standards. VIG units greatly reduce sound transmission, which improves the quality of living and working environments.
Because there is a vacuum between them, the glass sheets in a VIG unit are thermally isolated from one another to a far greater degree than those in inert gas units. As a result, the differential thermal strain between the glass sheets of a VIG unit caused by indoor and outdoor temperature differences in climates with large temperature extremes is far greater than for inert gas units. In a VIG unit with a rigid edge seal that joins both sheets of glass these differences in thermal strain meet at the unit's edges where they are constrained by compatibility. The result can be very large values of stress in the relatively non-ductile glass sheets and within the edge seal and its bond to the glass sheets.
The large stresses that can develop in the glass sheets of a VIG unit with a rigid edge seal can become so high that one or both ceramic glass sheets may fail suddenly in brittle fracture. This problem is exacerbated by ceramic glass's sensitivity to loss of strength from scratches and abrasions, which can precipitate breakage. If a VIG unit is a floor to ceiling window on the 94th floor of a building and fails suddenly in brittle fracture the consequences could exceed the cost of the unit's replacement and include injury or loss of life.
Although ceramic glass has a number of negative physical properties that are disadvantages in VIG construction, the lack of materials with its unique positive physical properties makes it very difficult to circumvent ceramic glass as the preferred transparent material for VIG units. The negative physical characteristics are brittleness, low ductility, low tensile strength, and a high a modulus of elasticity. The positive characteristics are very high rigidity, resistance to creep deformation under continuous loads, hardness, and very importantly ceramic window glass such as soda-lime glass has very low gas permeability. These positive properties make ceramic glass the preferred material for VIG units, which are subject to continuous flexural loads from atmospheric pressure and which must maintain service vacuum pressures for decades.
If ceramic glass was more ductile and had greater tensile strength then many of the problems plaguing VIG development would be greatly mitigated. Given that at present there is no suitable alternative to ceramic glass, the only available avenue for progress in VIG development is improved edge seal design. A number of United States patent application Publications disclose more flexible edge seal designs, which are attempts to mitigate many of the current problems with VIG performance, assembly, reliability, and safety.
In most of the VIG units described in the art the distance between the glass sheets is necessarily very much smaller than the distance between the glass sheets of inert gas filled thermal pane units and usually less than 0.08 inch. Despite the fact that close spacing of VIG unit glass sheets exacerbates the problem of accommodating differential thermal strain between them, close spacing of VIG unit glass sheets is desirable because spacers need to be small in order to be visually nonintrusive. Small spacers conduct less thermal energy. Close spacing of VIG unit glass sheets reduces the time required to pump down the vacuum, which reduces production costs. Spacers may be or include round disks, cylinders, micro sized particles, or even nanoparticles that may or may not be imbedded within the glass sheets.
In contrast to the typical distances between the glass sheets of VIG units, the distances between the glass sheets of inert gas units is chosen to minimize heat transmission from conduction and convection. That optimal spacing is between 0.625 and 0.75 inch. Because the distances between the glass panes of inert gas thermal pane widows are much greater than for VIG units, the stresses that develop in their edge seals are less than those for VIG units given the same lateral displacement between the glass sheets and similar sealing materials. Therefore the smaller differential thermal strains that develop between the glass sheets of inert gas units as compared to VIG units can be accommodated by simple flexible elastic seals that need not resist collapse under one atmosphere of pressure and that need not maintain a one atmosphere pressure difference for decades.
The rigid ceramic solder glass or glass frit edge seals that are currently used in VIG units and that are known in the art present serious problems. Seals of this type are disclosed by U.S. Pat. Nos. 5,664,395 and 5,657,607. The advantages of ceramic solder glass edge seals are their very low gas permeability and strong bond to ceramic glass substrates. Their disadvantage is brittleness and tendency to crack or fracture in climates with large temperature extremes such as occur in North America. It takes only a very small invisible crack or breach in a VIG edge seal to drastically reduce a unit's service life and to make repair infeasible.
In the process of forming rigid ceramic solder glass edge seals the ceramic glass sheets must be heated above a temperature that will remove tempering and introduce unwanted stresses within the glass sheets. The long heating and cooling times associated with this process increase manufacturing costs. The high assembly temperatures require the spacers to be of a material that can withstand those temperatures. This limits the range of suitable spacer materials and excludes materials with lower coefficients of thermal conductivity or higher creep resistance. U.S. Pat. Nos. 6,701,749; 6,558,494; 6,541,083; 6,641,689; 6,635,321; 6,478,911; 6,365,242; and 6,336,984 disclose methods that reduce the assembly temperatures of VIG units and allow the glass sheets to retain some but not all of their tempering.
Rigid edge seals can cause bulging out of a VIG unit's glass sheets. For example, if it is colder outdoors the outer glass sheet will contract causing both the inner and outer glass sheets to bulge inward toward the interior of the building increasing the likelihood of fracture. Bulging noticeably distorts reflections creating an objectionable non aesthetic fun house environment.
Nippon Sheet Glass produces commercial VIG units with ceramic solder glass edge seals under the trade name Spacia. U.S. Pat. Nos. 5,664,395, and 5,902,652 also describe such VIG units. Service information published for these units by Nippon Sheet Glass reveal many of the problems presented above. The service information states in part (Nippon 2003) (references cited: other publications):
Precaution for Use and Maintenance                1. When wired glass type is used in different application from conventional window, please contact us before use, to avoid of the trouble due to thermal breakage.        2. Don't paste the film and paper on SPACIA. It may brings about thermal breakage. Slight dislocation and occasional omissions of pillars, even if they are found, are negligible problem in terms of product performance.        3. SPACIA is required to use in temperature condition that its difference between IN and OUT is preferably less than 35° C.        4. Don't touch on SPACIA with metallic or ceramic hard sharp. Deep scratches sometimes lead to glass breakage.        5. Some deformation of reflective image is unavoidable for process reasons and for the occasional warpage of glass in case of a big temperature difference between IN and OUT, which is based on its higher thermal insulation. [sic]        
The problems associated with rigid edge seals can be reduced if a flexible seal is used. However, in comparison to stationary rigid seals, it is more difficult to achieve low permeability and leak rates for seals that accommodate or transmit motion. This difficulty exists for various reasons that include the following: flexible materials generally have higher gas permeability than rigid materials, and it is difficult to form lasting reliable bonds or tight fits between flexible elastic materials and the more rigid materials or configurations of vacuum vessels. The VIG edge seals disclosed by the United States Patent Application Publications discussed below are meant to be more flexible and ductile than rigid solder glass seals.
The problems with rigid ceramic solder glass edge seals and rigid edge seals for VIG units discussed above are enumerated by United States Patent Application Publications Nos. US 2008/01665570 A1 and US 2009/0155499 A1. These publications disclose designs that mitigate, but that do not eliminate, the above described problems by introducing metal as a bridging material between the edges of the glass sheets. Metal has greater ductility and flexibility than ceramic solder glass. This allows some movement of the edges of the ceramic glass sheets relative to one another under differential thermal strain. This results in less stress and likelihood of fracture. Some of the metal seals that are disclosed by the above publications are bent and folded into spring like forms that further increase their flexibility. These publications show some of the metal seals as being entirely between the glass sheets so that one of their dimensions is limited by the small distance between the glass sheets. This requires tight folds in the folded over metal forms and places limitations on the strains that can be accommodated without exceeding the elastic limit of the metals. Given the number of cycles of loading and unloading that would occur on a daily basis year after year because of expansion and contraction of the glass sheets, the metal seals disclosed by the above publications would very likely experience strain or work-hardening and become increasingly less ductile; possibly to a point where cracks or fissures would develop that would admit air into the vacuum at an unacceptable rate, shortening the service life of a VIG unit to years as opposed to decades. In regard to work-hardening of flexible metal joints that seal vacuums Jousten (2008, p 785) states (references cited: other publications):                For high—and ultrahigh—vacuum equipment, flexible metal elements are used, which are welded or brazed to the flanges. Such elements include hydraulically formed bellows (the longitudinal section is wavy) and diaphragm bellows (diaphragms, welded at the outside and inside perimeters). Because they are made of metal, every component of this type is subject to work-hardening and thus wear, depending on the number of working cycles.        
The folded over forms disclosed by the above publications are only effective as springs in one direction, whereas differential thermal strain in the glass sheets of a VIG unit occurs in two dimensions.
United States Patent Application Publication No. US 2009/0155499 A1 discloses that the contemplated metal edge seals may be bonded to the glass substrates by methods requiring lower temperatures than those required for solder glass seals. The methods and materials for bonding the metal strips to the glass substrates as disclosed by Pub. Nos. US 2008/01665570 A1 and US 2009/0155499 A1 are elastic in nature. Therefore the bond and bond material are subject to all the forces within the metal strips themselves. Those forces will be a function of the modulus of elasticity of the metal and the strain. Given seals made of elastic materials or having elastic bonds, any relative lateral displacement of the glass sheets will result in stresses that persist as long as the displacement persists. Under load, elastic materials are subject to failure from tensile rupture, shear rupture, stain hardening, and bond failure between joined elastic materials. Bond and material failure is a general problem with any primarily elastic material or bond used for sealing the edges of VIG units.
United States Patent Application Publication No. US 2010/0178439 A1 discloses a flexible edge seal for vacuum insulating glazing units. The preferred embodiment discloses a flexible edge seal consisting of a thin metal with convolutes. The seal is shown as being exterior to the space between the glass sheets of a VIG unit. The surface area of the seal as disclosed is very much greater than the surface area defined by the gap between the glass sheets. Two of the factors affecting rate of gas permeation are the surface area and thickness of the material through which gas permeates. The greater the surface area and the thinner the material through which gas permeates the greater will be the rate of permeation. In this regard the seal as disclosed by Pub. No. US 2010/0178439 A1 is less than optimal. The design of this seal requires a space, and therefore surface area, greater than the confines between the glass sheets will allow. The thin metal is bonded to the glass sheets and is therefore subject to both bond and elastic material failure modes.
United States Patent Application Publication No. US 2010/0034996 A1 discloses a flexible edge seal for vacuum insulating glazing units very similar to and with the same shortcomings as that disclosed by Pub. No. U.S. 2010/0178439 A1.